Heavy hydrocarbons can be converted into hydrocarbons within the average lower molecular weight by contacting these hydrocarbons under elevated temperatures with a cracking catalyst. During this operation a carbonaceous material often referred to as coke is deposited on the cracking catalyst. The coke reduces the activity of the cracking catalyst. In order to restore the activity, this cracking catalyst is introduced into a regenerator and the coke is burned off, e.g., with air. The regenerated cracking catalyst is then reintroduced into the catalytic cracking unit. The temperature in the catalytic cracker is of significant importance both with respect to the product made in the process and with respect to the quantity of coke deposited on the catalyst. The temperature in the cracking catalyst is, however, difficult to control.
Temperature control systems for controlling the temperature of the catalytic cracker have been described in the art. For instance the U.S. Pat. No. 3,828,171 shows measuring the temperature in a fluid catalytic cracking reactor bed and to control the flow of regenerated catalyst from the regenerator to the riser reactor. Two problems, however, remain to be solved. One is that the response to the temperature control is rather slow since the change achieved in the reactor vessel by the change of the flow of regenerated catalyst is delayed by the time it takes for the catalyst to reach the reactor vessel. A second problem is connected to the fact that the manipulation of a valve in the conduit through which the regenerated catalyst flows from the regenerator to the catalytic cracker is not linearly related to the temperature in the reactor. This non-linearity is related mainly to the valve itself in connection with the particular cracking catalyst material, but is also related to the slightly endothermic cracking reaction and the heat losses of the system.